This invention relates to medical ventilators and, more particularly, to an improved system for detecting an apnea condition in the delivery system for a medical ventilator.
In general, medical ventilator systems are used to provide respiratory support and anesthesia to patients undergoing medical treatment. The primary function of the ventilator is to maintain suitable pressure and flow of gases inspired and expired by the patient. Ventilator systems which are used in the administration of anesthesia to a patient undergoing an operation may include a bellows arrangement through which anesthetic gasses are provided to the patient. Alternatively, when used in ICU settings, the ventilator may provide flow directly to the patient.
The gasses actually delivered to the patient normally travel through a patient circuit that is generally a disposable unit having one end coupled to the patient via a face mask or other administration device and the other end is connected to the ventilator or to a bellows. According to the preference of the user, there are several differing types of patient circuits which can be used to deliver gasses to the patient. Perhaps the most common patient circuit is the standard circle system where there are separate limbs for providing the inhalation gas to a patient from the ventilator and for receiving the exhaled gases from the patient. Alternatively there may be a Bain circuit which again is commonly used and comprises one tubing enclosed within another or a modified Bain/Mapleson patient circuit where, again, there are coaxial tubes.
Among the normal monitoring functions of medical ventilators is the apnea alarm, that is, the alarm that signals to the user that breathing has ceased or has been reduced to an unacceptable level. Current apnea alarms are generally based on the volume of gas exhaled from the patient and therefore monitor the flow during the patient's exhalation through the use of a flow sensor. That flow is then integrated with respect to time to obtain a determination of exhalation volume and the apnea alarm signals if that exhaled volume is not of a predetermined value within a predetermined time period, indicating that the patient is not properly exhaling. As such, therefore, the conventional system uses a single flow sensor in the patient circuit at or near the patient in the expiratory limb of the patient circuit.
One of the difficulties with the use of a single flow sensor is that in the case of some circuit disconnects, i.e. a loss of pneumatic integrity of the ventilator system, it is possible to still see flow through the expiratory flow sensor, thus the apnea alarm will not always be triggered in a situation where an apnea condition exists. For example, there could be a partial patient disconnect that could be a problem in ventilating the patient, yet, if the threshold value that the flow sensor must see is relatively low, the flow sensor may see sufficient flow exhaled by the patient so that the apnea alarm would not be activated. Increasing the threshold value to correct this problem would negate the alarms efficacy when ventilating with small volumes.
Accordingly, the reliance on a single flow sensor that is positioned to sense the exhalation of the patient may not be sufficient to properly identify and signal an apnea condition that has taken place in the ventilator system.